Expandable interbody fusion device

ABSTRACT

An expandable interbody fusion device includes superior and inferior endplates that are configured to receive a sequentially inserted stack of interlocking expansion members or wafers. The like-configured wafers include features on their top and bottom surfaces that interlock the wafers in multiple degrees of freedom so that the wafer stack is not disrupted when the fusion device is fully expanded. One of the interlocking features includes a plurality of prongs projecting from an upper surface of the wafers and into a recess defined in the lower surface of an adjacent previously inserted like-configured wafer. The prongs and recesses are configured to prevent retrograde movement of each new wafer in a direction opposite the direction of insertion. Other interlocking features prevent movement in the direction of insertion, transverse to the insertion direction and vertically within the stack.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/736,514, filed Jan. 8, 2013, now U.S. Pat. No. 8,574,299, which is acontinuation of U.S. application Ser. No. 13/166,375, filed Jun. 22,2011, now U.S. Pat. No. 8,349,014, which is a continuation of U.S.application Ser. No. 11/756,050, filed May 31, 2007, now U.S. Pat. No.7,967,867, the contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to devices and methods for distraction andstabilization of tissue surfaces, and most particularly forstabilization of the intervertebral disc space in interbody fusionapplications.

The number of spinal surgeries to correct the causes of low back painhas steadily increased over the last several years. Most often, low backpain originates from damage or defects in the spinal disc betweenadjacent vertebrae. The disc can be herniated or can be suffering from avariety of degenerative conditions, so that in either case theanatomical function of the spinal disc is disrupted. The most prevalentsurgical treatment for these types of conditions has been to fuse thetwo vertebrae surrounding the affected disc. In most cases, the entiredisc will be removed, except for the annulus, by way of a discectomyprocedure. Since the damaged disc material has been removed, somethingmust be positioned within the intra-discal space, otherwise the spacemay collapse resulting in damage to the nerves extending along thespinal column.

In order to prevent this disc space collapse, the intra-discal space hasbeen filled with bone or a bone substitute in order to fuse the twoadjacent vertebrae together. In early techniques, bone material wassimply disposed between the adjacent vertebrae, typically at theposterior aspect of the vertebrae, and the spinal column was stabilizedby way of a plate or a rod spanning the affected vertebrae. With thistechnique, once fusion has occurred the hardware used to maintain thestability of the segment became superfluous. Moreover, the surgicalprocedures necessary to implant a rod or plate to stabilize the levelduring fusion were frequently lengthy and involved.

It was therefore determined that a more optimum solution to thestabilization of an excised disc space is to fuse the vertebrae betweentheir respective end plates, most optimally without the need foranterior or posterior plating. There have been an extensive number ofattempts to develop an acceptable intra-discal implant that could beused to replace a damaged disc and yet maintain the stability of thedisc interspace between the adjacent vertebrae, at least until completearthrodesis is achieved. These “interbody fusion devices” have takenmany forms, but many have had difficulty in achieving fusion, at leastwithout the aid of some additional stabilizing device, such as a rod orplate. Moreover, some of these devices are not structurally strongenough to support the heavy loads and bending moments applied at themost frequently fused vertebral levels, namely those in the lower lumbarspine.

The interbody fusion devices (IBFDs) that have overcome thesedifficulties are typically bulky, at least with respect to theintervertebral space. In particular, these devices have been configuredto completely fill the space and to restore the normal spinal anatomy atthe instrumented level. One drawback of this approach is that theimplant device is not exactly sized to the anatomy of the particularpatient, thus typically requiring pre-distraction of opposed vertebraein order to increase the disc space for device implantation. While acollection of differently sized IBFDs can be provided, it is unwieldyand impractical to provide an IBFD sized for every intervertebral discspace height.

Another drawback of these prior devices is that the surgical insertionsite must be at least as big as the IBFD. Minimally invasive and workingchannel surgical techniques have been recently developed that havesignificantly reduced the surgical invasion, but even more improvementis needed. One solution to these drawbacks was presented in U.S. Pat.No. 6,595,998 (the '998 patent), entitled “Tissue Distraction Device”,which issued on Jul. 22, 2003, to the assignee of the present invention.The '998 patent discloses sequentially introducing a series of wafersinto the space (whether inter- or intra-vertebral) using a percutaneousintroducer. In certain embodiments, the wafers included features thatallowed adjacent wafers to interlock to some degree along thelongitudinal axis of the wafers. The disclosure of the '998 patent isincorporated herein by reference, particularly as it pertains to theinterlocking features of the wafers and the percutaneous introducer.

In an improvement on the wafer concept in the '998 patent, an expandabledistraction device was disclosed in co-owned pending application Ser.No. 10/813,819 (the '819 Application), which was filed on Mar. 31, 2004,and published as Pub. No. 2005/0187559 on Aug. 25, 2005. The disclosureof the '819 Application is incorporated herein by reference. Theexpandable distraction device disclosed in the '819 Application includesa plurality of wafers that are successively inserted to form a stack ofwafers in a column. The wafers are configured so that a newly insertedwafer lifts the stack of previously inserted wafers, including thesuperior endplate, until the space has been distracted to a desiredheight.

A further improvement is disclosed in co-owned pending application Ser.No. 11/211,346 (the '346 Application), which was filed on Aug. 25, 2005,and was published as Pub. No. 2006/0058807 on Mar. 16, 2006. Thedisclosure of the '346 Application is incorporated herein by reference.The '346 Application discloses a wafer insertion apparatus 50 thatincludes a wafer track 52 configured at one end to releasably engage theinferior endplate of the expandable device 10, as depicted in FIG. 1.The other end of the wafer track is connected to a gun 51 that supportsa cartridge of wafers 54 and includes a trigger-operated mechanism 53for extracting a wafer from the cartridge and advancing it along thewafer track into the wafer cavity between the superior and inferiorendplates. The wafer insertion apparatus initially supports theexpandable device in situ and includes a release plate operable toseparate the wafer track from the expandable device when wafer insertionis complete.

The wafer insertion apparatus disclosed in the '346 Application utilizesa series of posts as shown for example in FIGS. 44-45 thereof formed inthe wafer cavity defined by the inferior endplate. The posts are engagedby an insertion plate that forms part of the wafer track so that thetrack can support the expandable distraction device in situ during waferinsertion. A release plate severs the posts to allow the wafer track todisengage the inferior endplate for removal of the wafer insertionapparatus.

The wafers disclosed in the '346 Application include features thatfacilitate interlocking between adjacent wafers. Thus, as illustratedfor example in FIGS. 28-29 and FIGS. 35-36 of the '346 Application, thewafers include resiliently deflectable features that deflect and lockupon longitudinal insertion of a new wafer underneath a previouslyinserted wafer.

In preferred uses of the expandable devices described above, it iscontemplated that bone promotion filler such as osteoinductive orosteoconductive material may be integrated around, and in some casesinto, the stack of wafers forming the distraction device. Ideally, oncefusion occurs the entire space is rigid, as if the entire space is bone.In an interbody fusion procedure, the vertebrae adjacent the affecteddisc space are fused together so that the motion segment is eliminatedat the disc level. The distracted space is subjected to significantloads, even when efforts are made to immobilize the spine around theaffected vertebral level. While the compressive loads along the lengthof the spine are readily borne by the expanded distraction device andassociated wafer stack, transverse loads and most particularly torsionloads must also be withstood.

Consequently, there remains a need for an expandable distraction devicethat can endure the significant spinal loads and maintain suitablestructural integrity, at least until complete fusion can be achieved.

SUMMARY OF THE INVENTION

In order to address these objectives, the present invention contemplatesa device for distracting a body tissue space between opposing tissuesurfaces, comprising an upper plate having an outer surface configuredto contact one of the opposing surfaces and a lower plate having anouter surface configured to contact the other of the opposing surfaces.The upper and lower plates combine to define a cavity when the upperplate is supported on the lower plate. The lower plate includes asupport surface for supporting at least one expansion member, or wafer,within the cavity, and a channel communicating with the cavity that isconfigured to receive an expansion member conveyed therethrough forplacement on the surface of the lower plate.

In one embodiment, the wafers are like-configured, each comprising aninterlocking engagement that includes a plurality of resilientlydeflectable prongs that project above an upper surface of the wafer, anda like plurality of locking surfaces extending transversely between theupper surface and the lower surface. The prongs deflect as each wafer isinserted into the space between the expandable upper and lower platesuntil each wafer is substantially co-extensive with the prior insertedwafer. When so oriented, the prongs resiliently deflect upward againstcorresponding locking surfaces to prevent retrograde movement (i.e.,opposite the direction of insertion) of the newly inserted wafer.

Each wafer is configured with angled leading and trailing ends so thateach newly inserted wafer lifts the stack of prior inserted wafers. Aseach wafer is inserted, additional interlocking features are engagedthat prevent movement of the wafers in other degrees of freedom. Oneinterlocking feature includes a keyway and tab arrangement that preventsfurther movement along the direction of insertion. Yet anotherinterlocking feature prevents relative movement transverse to theinsertion direction and vertically within the stack of wafers. Theinterlocking features also prevent disengagement of the wafers ordislodgement of any wafer from the stack due to torsional or twistingmovement.

In one embodiment, an expansion member for sequential insertion into aspace between opposing tissue surfaces to be distracted is provided thatcomprises an elongated body having an upper surface and an oppositelower surface, at least one locking surface extending transverselybetween the upper surface and the lower surface, and at least oneresilient prong defined in and projecting outwardly beyond one of theupper surface and the lower surface. In one feature, the at least oneprong is oriented such that when two of the expansion members areimmediately adjacent and substantially co-extensive at least a portionof the at least one prong of one expansion member is disposed in contactagainst the locking surface of the other expansion member.

In another embodiment, an expandable interbody fusion device (IBFD) forimplantation into the intradiscal space between two opposing vertebralbodies of a spine, comprises a first endplate member having an outersurface for contacting one vertebral body in a spine, and a secondendplate member having an outer surface for contacting an opposingvertebral body in the spine, the second endplate member being movable inan expansion direction relative to the first endplate member toward theopposing vertebral body. The IBFD further comprises an expansion memberconfigured to be introduced between the first endplate member and thesecond endplate member in an insertion direction that is substantiallyperpendicular to the expansion direction, to thereby move the first andsecond endplate members relatively apart in the expansion direction uponintroduction, and an interlocking prong and cavity engagement definedbetween the expansion member and the second endplate member to preventrelative movement of the expansion member relative to the secondendplate member in a direction opposite the insertion direction when theexpansion member is substantially co-extensive with the second endplatemember. In one aspect of this embodiment, the expansion member includesa surface facing the second endplate member and the second endplatemember has a surface facing the expansion member, and the cavity isdefined in the facing surface of the second endplate member and theprong projects beyond the facing surface of the expansion member intothe recess when the expansion member is substantially co-extensive withthe second endplate member.

It is one object of the invention to provide an improved expandabledevice that may be used to distract the space between two body tissuesurfaces. A further object of the invention is to provide expansionmembers that interlock in multiple degrees of freedom.

One benefit of this feature is that the wafers become interlocked uponsequential insertion. Other objects and benefits of the invention willbecome apparent upon consideration of the following written descriptiontaken together with the accompanying figures.

DESCRIPTION OF THE FIGURES

FIG. 1 is a side view of an expandable distraction device mounted on awafer insertion apparatus as disclosed in co-pending publishedapplication No. 2006/0058807.

FIGS. 2 and 3 are rear and front perspective views of an expandabledistraction device comprising a stack of wafers in accordance with oneembodiment of the present invention.

FIGS. 4 and 5 are side and end views of the expanded device shown inFIGS. 2-3.

FIG. 6 is an end cross-sectional view of an expandable distractiondevice such as the device shown in FIGS. 2-5 shown with a single wafertherein prior to expansion of the device.

FIG. 7 is an end cross-sectional view of the expandable distractiondevice depicted in FIGS. 2-5 showing expansion by the introduction of asecond wafer into the device.

FIG. 8 is a perspective view of an interlocking wafer in accordance withone embodiment of the present invention that is configured to form awafer stack within an expandable distraction device, such as the deviceshown in FIGS. 2-5.

FIG. 9 is a top view of the interlocking wafer shown in FIG. 8.

FIG. 10 is a side view of the interlocking wafer shown in FIGS. 8-9.

FIG. 11 is an insertion end view of the interlocking wafer shown inFIGS. 8-10.

FIG. 12 is a trailing end view of the interlocking wafer shown in FIGS.8-10.

FIGS. 13-14 are perspective and plan views of the bottom of theinterlocking wafer shown in FIGS. 8-10.

FIG. 15 is a longitudinal perspective cross-sectional view of theinterlocking wafer depicted in FIGS. 8-14.

FIG. 16 is an end cross-sectional view of the expandable distractiondevice depicted in FIGS. 6-7 shown with a three-wafer stack within thedevice after removal of the wafer track.

FIG. 17 is a side cross-sectional view of an expandable distractiondevice, such as the device shown in FIGS. 2-5, with a stack ofinterlocking wafers of the present invention disposed within the device.

FIG. 18 is an exploded side view of a wafer inserter apparatus and awafer cartridge suitable for introducing interlocking wafers of thepresent invention into an expandable distraction device.

FIG. 19-20 are a top and bottom perspective views of an alternativeinterlocking wafer in accordance with a further embodiment of theinvention.

FIGS. 21-22 are top and bottom perspective views of another interlockingwafer in accordance with a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiments illustrated inthe drawings and described in the following written specification. It isunderstood that no limitation to the scope of the invention is therebyintended. It is further understood that the present invention includesany alterations and modifications to the illustrated embodiments andincludes further applications of the principles of the invention aswould normally occur to one skilled in the art to which this inventionpertains.

The present invention contemplates an improved interlocking wafer, andparticularly a wafer configuration that firmly and permanentlyinterlocks a stack wafers inside an expandable distraction device, evenwhen subjected to normal spinal loads. In accordance with one embodimentof the invention, an expandable distraction device 250 is provided, asshown in FIGS. 2-5, which includes a stack of interlocking wafers 300that can withstand spinal loads. The distraction device 250 includes asuperior endplate 251 and an inferior endplate 252 that may be similarto the endplates disclosed in the '346 Application, which disclosure isincorporated herein by reference. The surfaces of the endplates includeridges 254 that are adapted to firmly grip the vertebral bodies when thedevice is expanded to distract the intervertebral space. In theillustrated embodiment, the contours of the endplates are adapted toengage the bony endplates of the adjacent vertebrae.

The inferior plate 252 defines a wafer channel 257 through which thewafers 300 which serve as expansion members are introduced. As with theendplates disclosed in the '346 Application, the inferior endplatedefines opposite ledges 260 which support each wafer as it is introducedinto the wafer channel, as illustrated in FIG. 7. The inferior endplatealso defines an inserter channel 258 that is underneath and incommunication with the wafer channel 257. The inserter channel 258receives a wafer track, such as track 52 shown in FIG. 1. Morespecifically the inserter channel 258 includes a number of posts 262projecting upward therein that are configured, as illustrated in FIGS.6-7, to engage an insertion plate 270 and a release plate 272 in amanner similar to that described in the '346 Application incorporatedherein.

The superior and inferior endplates 251 and 252 are configured to beinitially releasably engaged when the device 250 is unexpanded, as shownin FIG. 6. In one embodiment, each opposite side wall 264 of theinferior endplate 252 defines a pair of ribs 265 projecting into thewafer channel, the ribs 265 being spaced lengthwise on each side wall264. The superior endplate 251 includes a hub portion 269 that is sizedto fit within the wafer channel 257 and between the side walls 264 ofthe inferior endplate. The hub portion 269 defines a groove 280extending along each side of the hub portion that is configured toengage the ribs 265 of the inferior endplate. This engagementtemporarily holds the superior and inferior endplates together as thedevice 250 is introduced into the space to be distracted. The hubportion 269 further defines in a particular arrangement a pair of spacednotches 282 beneath the groove 280 that are also configured to receivethe corresponding pair of ribs 265. As shown in FIG. 7, with the ribs265 extending into the notches 282, the superior endplate 251 is freelyseparated from the side walls 264 of the inferior endplate 252 and thelower surfaces of the ribs 265 contact the angled upper edges 315 (seeFIG. 11) of the side wall 314 of wafer 300 a. As such, the ribs 265provide some resistance against wafer 300 a until further wafers areintroduced as seen in FIG. 16.

Details of one embodiment of the interlocking wafer 300 are shown inFIGS. 8-14. The wafer 300 has an upper surface 302 and a lower surface303, both of which are generally planar so that the wafers can form astable stack within the IBFD 250. The trailing end 305 includes adownward-facing sloped surface 306 that corresponds angularly to anupward-facing surface 309 on the leading end 308 of the wafer. The twosloped surfaces help displace an earlier inserted wafer 300 uponintroduction of a new wafer. More specifically, when a wafer is withinthe wafer channel 257, resting on the ledge 260 (FIG. 7), thedownward-facing sloped surface 306 is lifted by contact with theupward-facing slope 309 of a newly inserted wafer. This allows the newlyinserted wafer to ride along the ledge 260 until it is positioned fullyunderneath the previous wafer.

The wafer 300 further includes notches or indentations 312 that areconfigured to receive the ribs 265 on the side walls of the inferiorplate 251 (see FIG. 16) in a manner similar to the notches 282 in thehub portion of the superior endplate 251. In the preferred embodiment,the indentations 312 are offset toward and intersect the lower surface303. As best seen in FIG. 16, the ribs 265 sit within the indentations312 of an “upstream” wafer 300 a and bear against an upper angled edge315 of the side walls 314 of the “downstream” wafer 300 b underneath. Inthe illustrated embodiment, two spaced ribs 265 are provided on eachside wall of the inferior endplate. Thus, the hub portion of thesuperior endplate 251 and the wafers 300 include two correspondingnotches 312 oriented to receive the ribs. Of course, different numbersof ribs and notches may be provided.

The wafer 300 includes several features to interlock adjacent wafers inmultiple degrees of freedom. One particular feature includes a series ofresiliently deflectable prongs 320 that project outwardly above theupper surface 302 of the wafer (as best seen in FIGS. 11-12). In onearrangement, the prongs 320 are disposed generally centrally on thecentral portion 333 of each wafer 300, extending lengthwise inalignment. Each prong 320 is seated within a cavity 322 defined throughthe wafer. Each prong is cantilevered from an adjoining wall 325 betweencavities, as best seen in FIGS. 15 and 17, so that the prong can deflectinto the cavity upon pressure on the prong from above the wafer. Thecavity 322 includes a rear ledge 323 at the lower surface 303. Incertain embodiments, the rear ledge may extend sufficiently far into thecavity beneath the prong 320 to keep the prong from bending underneaththe wafer. More importantly, the rear ledge 323 defines a stop surface324 at the lower surface 303 of the wafer against which the cantileveredface 321 of an associated prong bears. In the illustrated embodiment,five prongs 320 and associated stop surfaces 324 are provided on eachwafer to provide a firm interlocking engagement between adjacent wafers.Of course, it is contemplated that fewer or greater numbers of prongsmay be provided in a wafer within the scope of the present invention.For instance, the number of prongs may be adjusted based on the lengthof the wafer 300.

Thus, as shown in FIG. 17 an uppermost wafer 300 a provides a stopsurface 324 a that is contacted by the cantilevered face 321 b of thenext lower wafer 300 b as the prong 320 b of that lower wafer projectsinto the cavity 322 a of the upper wafer. It can be appreciated thateach of the five prongs 320 b of the wafer 300 b shown in FIG. 17project into a corresponding cavity 322 a and engage the associated stopsurface 324 a to lock the wafers against retrograde movement—i.e.,movement opposite the direction of insertion I—that might lead toexpulsion of the wafer from within the expanded device 250. It should beappreciated that as each subsequent wafer is sequentially inserted, theassociated prongs 320 b deflect downward against the associated ledge323 b as the prongs progressively traverse the lower surface 303 a of apreviously inserted wafer 300 a. Preferably, the prongs are providedwith an angled surface 326 that bears against the underside of theprevious wafer and progressively deflects the prong as the angledsurface traverses the other wafer. Once the prongs in the lower wafer300 b are aligned with the cavities 322 a in the upper wafer, the prongsspring upward into the cavity 322 of the previous wafer to positivelyand substantially permanently lock the two wafers together in thedirection of insertion I.

As shown in FIG. 17, each wafer in the stack locks into the immediatelypreviously inserted wafer using the prongs and stop surface. Theuppermost wafer 320 a also engages the superior endplate 251 to lock thestack to the expandable device 250. Thus, in one embodiment, a stopsurface 328 is formed in corresponding recess 329 defined in theunderside of a hub portion 269 of the superior plate 251. The hub 269 isalso provided with a downward-facing sloped surface 330 similar to thesloped surface 306 of the wafers, to facilitate introduction of theinitial wafer 300 a and to lift the superior endplate as that initialwafer is introduced. Thus, it can be seen that the superior plate 251and the successively inserted stack of wafers 300 are all interlockedagainst longitudinal movement opposite the direction of insertion I.

The cooperating locking structure of the wafers 300 also restricts orprevents movement of the wafers in the stack in the direction ofinsertion I. In one embodiment, the upper surface 302 defines a keyway335 with side channels 336 and an upper wall 337, as shown in FIGS. 11,15 and 17. The lower surface 303 defines a complementary notch 340 andtab 341, as shown in FIGS. 13-14. The tab 341 fits within the sidechannels 336 and the notch engages the upper wall 337 of the keyway, asseen in FIG. 17. Thus, the tab and keyway prevent longitudinal movementof each wafer relative to the previously inserted wafer in the directionof insertion I. As shown in FIG. 17, there may be some clearance Cbetween the cantilevered face 321 of the prongs 320 and thecorresponding stop surfaces 324 of the wafers. The overlappingengagement between the keyway 335 and the notch 340 and tab 341 issufficient so that any retrograde movement of the wafers that closesthis clearance will not disengage the tab from the keyway. It iscontemplated that the clearance C between prongs and stop surfaces maydecrease from the trailing end 305 to the leading end 308 of the wafer,as depicted in FIG. 17, to enhance the rigidity of the engagementbetween wafers.

As thus far described the wafer stack is locked against movement in thelongitudinal direction (i.e., fore and aft relative to the insertiondirection I). Certain embodiments of the locking structure describedherein further contemplate restricting or preventing relative movementbetween wafers in multiple degrees of freedom. Thus, in one embodiment,the upper surface 302 includes a channel 350 formed at each lateral sideof the wafer 300 flanking the central portion 333 that carries theprongs 320, as seen in FIGS. 8-9. The central portion 333 extends into aflange 334 overhanging each channel that forms a groove 351 contiguouswith the channel 350 (FIG. 11) thereby defining a T-bar configuration).The channel and groove on each side of the wafer extends from theleading end 308 to a stop face 352 adjacent the trailing end 305.

The bottom surface 303 of each wafer defines features for mating withthe T-bar configuration on the upper surface 302 of a successive wafer.Thus, as shown best in FIGS. 13 and 16, the bottom surface includes acenter track 357 with side flanges 358 that correspond define a T-slotconfiguration which corresponds to and slidably mates with the T-barconfiguration on the upper surface of an immediately adjacent wafer 300,with flanges 334 being received in the track 357. It should beappreciated that the respective T-bar and T-slot configurations may beformed on either the upper surface or the lower surface of a wafer asdesired. This interlocking relationship restricts or prevents transverseor lateral movement of one wafer relative to adjacent wafers. It canalso be appreciated that the interaction between the track 357 and sideflanges 334 also restricts or prevents vertical separation betweenwafers. This interlocking engagement occurs automatically when one waferis introduced into the IBFD 250 underneath and substantiallyco-extensive lengthwise with a previously inserted wafer. The stop face352 stops the linear advancement of one wafer relative to the other whenthe end surface 359 (FIG. 13) abuts the stop face. The length of thechannel is calibrated so that the end faces 359 of the underneath waferreach the stop faces 352 after the prongs 320 have engaged thecorresponding cavities 322 in the immediately preceding wafer.

As shown in FIG. 7, the lowermost wafer 300 b rests on the wafer supportledges 260 defined in the inferior plate 252. As more particularlydisclosed in the '346 Application, the wafers are directed onto thesesupport ledges by passage along a wafer track assembly, such as thetrack assembly 52 shown in FIG. 18. In the view shown in FIG. 7, theposts 262 are intact and an insertion plate 270 is shown within theinsertion channel 258 with the posts 262 projecting throughcorresponding holes 271 in the plate. The insertion plate extendsthrough the wafer track 52 for engagement with the mechanism of theinserter gun 600 (FIG. 18). In one embodiment, the openings 271 areconfigured with a cutting edge so that upon withdrawal of the insertionplate, the posts 262 are severed. Alternatively, a release plate 272 maybe provided underneath the insertion plate, as shown in FIGS. 6-7. Inthis instance, the release plate is retracted beneath the insertionplate to sever the posts. Once the posts have been severed, the trackassembly 52 may be disconnected from the completed distraction device250. The resulting IBFD 250 with the stacked wafers 300 appears as shownin FIGS. 2-5, and in FIGS. 16-17.

The manner in which the IBFD 250 is formed is illustrated in thesequence shown in FIGS. 6, 7, 16 and 17. In FIG. 6 the superior endplate251 is shown with one wafer 300 a already engaged to the hub 269. Inthis condition, IBFD 250 is unexpanded and is attached to the trackassembly 52 as depicted in FIG. 18. In one embodiment, the expandabledevice is initially provided with this wafer in place and the superiorand inferior endplates releasably connected by way of the groove 280 andribs 265. When the first inserted wafer 300 b is introduced into thedevice, the newly inserted wafer lifts the first wafer 300 a and thesuperior endplate 251 until the angled edges 315 of the side walls 314of the first inserted wafer 300 b contact the ribs 265, as shown in FIG.7. When a second wafer 300 c is inserted, as shown in FIG. 16, the firstwafer 300 a is pushed above the inferior endplate, while which theangled edges 315 of the second wafer 300 b now engage the ribs. Thisprocess continues with each successively inserted wafer until a completestack if formed, as depicted in FIG. 17. It should be appreciated thatthe trailing end of a previously inserted wafer may tend to elevatebefore the insertion end as a successive wafer is inserted, especiallywhere the wafers are relatively rigid. In such situation, the ribs 265adjacent the front end will continue to apply resistance to the frontend of the forming wafer stack even if the back end is initiallyseparated from the ribs 265 at the back end of the stack.

The wafers may be incorporated into a cartridge 650 that is adapted toreleasably fit into a wafer insertion device 600 shown in FIG. 18. Thedevice 600 may incorporate the same internal mechanism within housing602 that is incorporated into the wafer insertion device disclosed inthe '819 Application incorporated by reference above. Specifically, themechanism is operable to withdraw a wafer from the cartridge and propelsuccessive wafers along wafer track assembly 52 into an IBFD 300 that ispreloaded onto the end of the track assembly. It is understood that thedevice 600 is preferably a reusable instrument, while the cartridge 650may be reusable or disposable.

The wafer insertion device 600 includes a gun housing 602 that defines atop opening 604 for receiving the cartridge 650. The cartridge includesa pair of vertical ribs 652 on both sides of the cartridge that slidablymate with corresponding internal grooves 606 formed in the gun housing602. The cartridge is further provided with a resilient latch 654 with acatch end 656 on each side of the cartridge that engages the housing602. The latch can be manually depressed to release the cartridge fromthe gun when the IBFD 300 has been fully loaded with a stack of wafers.The cartridge 650 includes a wafer housing 658 that supports a supply ofwafers, and a track housing 660 projecting from the wafer housing. Thetrack housing 660 may incorporate portions of the advancement mechanismand guide tracks disclosed in the '819 Application incorporated byreference. The trigger 610 operates the mechanism to extract and advancea wafer along the track 52. The second trigger 612 is connected to theinsertion plate 270 (or alternatively the release plate 272) to retractthe plate when the wafer stack is complete in order to sever the posts262, as described above.

It can be appreciated that the interlocking wafer 300 of the presentembodiment provides for interlocking engagement that prevents orsignificantly restricts relative movement in multiple degrees offreedom, including longitudinally, transversely and vertically relativeto the wafer body. In addition, the engagement between the prongs andthe stop surfaces is maintained even when the wafers are subject totorsion either along the longitudinal axis of the wafer or along aperpendicular axis. All of the interlocking structural features arecalibrated to automatically engage once a newly inserted wafer is fullyaligned beneath the previously inserted wafer.

The rigid interlocking engagement as described is provided to preventdislodgement of any wafer in the stack when the expanded IBFD issubjected to the normal spinal loads. Filler material may be introducedinto the space surrounding the expanded device 250, such that the fillermaterial in conjunction with the expanded device 250 will form a rigidstructure between the adjacent bone surfaces. Thus, once this rigidstructure is created (such as by hardening of a filler material orfusion of natural bone within the space) the spinal loads are borne bythe entire rigid structure.

In an alternative embodiment, an interlocking wafer 400 is substantiallysimilar in construction to the wafer 300, except that the wafer 400shown in FIGS. 19-20 eliminates the keyway 335 of the prior wafer. Thewafer 400 includes multiple prongs 420 that are configured and operatethe same as the prongs 320 of the prior embodiment. In lieu of thekeyway configuration, the wafer 400 reverses the orientation of theforward-most prong 422 so that the forward portion 423 engages a stopface 428 of the recess 425 surrounding the prong. Thus, it can be seenby comparing the prong 422 to the other prongs 420 that the forward-mostprong faces in the direction of insertion, rather than opposite, and isconfigured to deflect upward, rather than downward, as a subsequentwafer is inserted underneath. Once the subsequent wafer is disposedfully underneath the wafer 400, the forward-most prong 422 snaps intothe recess 425 of the underneath wafer. The interaction between theforward portion 423 and the stop surface 428 prevents further forwardmovement of the upper wafer relative to the underneath wafer in theinsertion direction I.

In another embodiment, an interlocking wafer 500 is provided as shown inFIGS. 21-22. This interlocking wafer includes a generally centrallydisposed flexible strip 502 that includes a series of sloped ridges 504on an upper face 505 of the strip and a complementary offset series ofsloped ridges 508 on a lower face 506 of the strip. The strip isrecessed from the upper surface 510 of the wafer 500 within a cavity 512formed through wafer. The ridges 508 project below the bottom surface514 of the wafer so that they can engage the upward-facing ridges 504.The flexible strip 502 resiliently flexes as the upward-facing anddownward-facing sloped ridges pass over each other until the end faces515 of the ridges 504 are abutting the end faces 516 of the lower ridges508. The abutting end faces 515 and 516 prevent retrograde movementbetween the two wafers.

The wafer 500 may incorporate other interlocking features found in thewafer 300 that limit transverse and superior-inferior movement ofadjacent wafers. In an additional feature, the side walls 515 mayincorporate wafer removal features 518. In the illustrated embodiment,these wafer removal features are in the form of ridges 519 with pockets520 between the ridges that are formed for access by a suitable tool forremoval of a wafer if necessary.

It is contemplated that each of the wafers 300, 400, and 500 describedherein is formed of a biocompatible material that is sufficiently rigidto form a solid stack within the expandable distraction device, but thathas sufficient resilient properties for the prongs to deflect undermanual pressure as the successive wafers are inserted into the device.Thus, in one specific embodiment, the wafers are formed of PEEK or acarbon-fiber reinforced PEEK, or similar polymeric material. Preferably,the material is suitable for forming the wafers in a molding process,with little or no machining required to create the various features ofthe wafers. As an alternative, the prongs 320 and adjoining wall 325 maybe formed of a resilient material with the remainder of the wafer 300being over-molded with a different material that does not require theresilient properties of the prongs. The superior and inferior plates251, 252 are also formed of a biocompatible material, which may be thesame as the wafers. Alternatively, the superior and inferior plates maybe formed of a biological material, such as a bone graft material, or anosteoconductive or osteoinductive material

The wafers may be formed from a solid form of bone filler material,and/or any other suitable material such as, but not limited to,implantable grade alloys, medical grade composites, medical gradepolymers, ceramics, hydrogels and resorbable polymers. The wafers may bedense or porous, while porous wafers may be filled with resorbablepolymers, drug therapies or osteoinductive agents.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same should be considered asillustrative and not restrictive in character. It is understood thatonly the preferred embodiments have been presented and that all changes,modifications and further applications that come within the spirit ofthe invention are desired to be protected. For instance, while theillustrated embodiments have been directed to interbody fusion of thespine, the expandable devices and wafers disclosed herein may be used inother applications that require distraction of tissue surfaces.Modifications in size may be necessary depending upon the body spacebeing distracted.

For example, the prongs 320 of the wafer 300 may be oriented to projectbelow the lower surface 303 of the wafer. With this orientation, thepassage of a wafer underneath a previously inserted wafer willresiliently deflect the downward projecting prongs until all of thecavities 322 align with all of the prongs. If the orientation of theprongs is altered, it is necessary to also alter the hub 269 of thesuperior plate 251 of the IBFD 250 to include downward projecting prongsin lieu of the recesses 329. Moreover, as suggested by the wafer 400shown in FIGS. 19-20, the prongs on the wafer 300 may be oriented toproject above and below the wafer.

In the wafer 300, the prongs 320 are situated within cavities 322 thatextend through the wafer. Alternatively, prongs may be disposed within acavity that does not pass through the thickness of the wafer. In thisinstance, the stop surface 324 may defined in a separate recess formedin the lower surface 303 of the wafer.

What is claimed is:
 1. An expandable interbody fusion device forimplantation into the intradiscal space between two opposing vertebralbodies of a spine, comprising: a first endplate member having an outersurface for contacting one vertebral body in a spine; a second endplatemember having an outer surface for contacting an opposing vertebral bodyin said spine, said second endplate member being movable in an expansiondirection relative to said first endplate member toward the opposingvertebral body; at least one insert configured to be introduced betweensaid first endplate member and said second endplate member in aninsertion direction that is substantially perpendicular to saidexpansion direction; and cooperating locking structure between saidinsert and said first endplate member and said second endplate member torestrict relative movement in multiple degrees of freedom, including atleast one interlocking element that is resiliently deflectable in theexpansion direction.
 2. The expandable interbody fusion device of claim1, wherein said at least one insert comprises an upper surface and alower surface and wherein said interlocking element comprises aresiliently deflectable prong projecting outwardly from one of saidupper surface and said lower surface.
 3. The expandable interbody fusiondevice of claim 2, wherein said cooperating locking structure comprisesa cavity extending into said second endplate member and a lockingsurface communicating with said cavity, said resiliently deflectableprong projecting into said cavity and engaging said locking surface. 4.The expandable interbody fusion device of claim 3, including at leasttwo inserts of substantially identical configuration, said inserts beingdisposed between said first endplate member and said second endplatemember in the expansion direction.
 5. The expandable interbody fusiondevice of claim 4, wherein each insert further includes a cavity and alocking surface extending into the other of said upper surface and saidlower surface from which said prong projects, a prong on one insertextending into the cavity and engaging the locking surface of the otherinsert.
 6. The expandable interbody fusion device of claim 5, whereineach insert includes a plurality of prongs and cavities and lockingsurfaces extending generally centrally along the length of each saidinsert.
 7. The expandable interbody fusion device of claim 6, whereineach prong and cavity is spaced along each insert so that upon slidingcontact of one insert lengthwise relative to the other insert, saidprongs on one insert progressively engage the locking surfaces on theother insert.
 8. The expandable interbody fusion device of claim 4,wherein said cooperating locking structure further includes a T-barconfiguration on one insert and a T-slot configuration on the insertshaped to slidingly receive said T-bar configuration upon said slidingcontact of said inserts.
 9. An expandable interbody fusion device forimplantation into the intradiscal space between two opposing vertebralbodies of a spine, comprising: an elongate first endplate member havingan outer surface for contacting one vertebral body in a spine andcomprising opposed spaced sidewalls and opposed spaced front and rearendwalls defining therewithin an interior cavity, an upper supportsurface within said cavity, and a fully bounded channel opening throughsaid rear endwall in communication with said interior cavity; anelongate second endplate member having an outer surface for contactingan opposing an opposing vertebral body in said spine and a lower surfacehaving a locking configuration, said second endplate member beingmovable in an expansion direction relative to said first endplate membertoward the opposing tissue surface; a first insert sized to be initiallyslidingly received into said device and supported on said upper supportsurface between said first endplate member and said second endplatemember, said first insert comprising an upper surface including at leastone interlocking element that is resiliently deflectable in theexpansion direction and that upon receipt into said device resilientlyinterlocks with the locking configuration of the lower surface of saidsecond endplate member, said first insert comprising a lower surfacehaving a locking configuration substantially similar to the lockingconfiguration of said second endplate member; and a second insert sizedto be slidingly received into said device subsequent to said firstinsert and supported on said upper support surface between said firstinsert and said first endplate member, said second insert comprising anupper surface including at least one interlocking element that isresiliently deflectable in the expansion direction and that upon receipttherein resiliently interlocks with the locking configuration of thelower surface of said first insert, said second insert comprising alower surface having a locking configuration substantially similar tothe locking configuration of said first insert; said first insert andsaid second insert being disposed between the opposing sidewalls of saidfirst endplate member.
 10. The expandable interbody fusion device ofclaim 9, further including a flange extending along a longitudinalportion of one of said first insert and said second endplate member anda groove extending along a longitudinal portion and within the other ofsaid first insert and said second endplate member, said groove beingsized and configured to receive said flange therewithin as said firstinsert is slidingly received between said first endplate member and saidsecond endplate member.
 11. The expandable interbody fusion device ofclaim 10, wherein said flange defines a T-bar configuration and saidgroove defines a T-slot configuration to slidingly receive said T-barconfiguration upon said sliding introduction of said expansion member.12. The expandable interbody fusion device of claim 11, wherein saidinterlocking element of each of said first insert and said second insertcomprises a resiliently deflectable prong projecting outwardlyrespectively from one of said upper surface and said lower surface. 13.The expandable interbody fusion device of claim 12, wherein each insertfurther includes a locking cavity and a locking surface extending intothe other of said upper surface and said lower surface from which saidprong projects, a prong on said second insert extending into the lockingcavity and engaging the locking surface of the first insert.
 14. Theexpandable interbody fusion device of claim 13, further including a stopface defined on one of said upper surface and said lower surface of saidfirst insert and said second insert.